1. Field of the Invention
The invention relates generally to a system for compensating electromagnetic interfering fields, and in particular to a system for magnetic field compensation having two sensors and a digital processor.
2. Description of Related Art
For compensating electromagnetic interfering fields, in particular magnetic interfering fields, feedback control systems are used in the very most cases, whereby one, or more sensors measure the amplitude of the interfering field for all three Cartesian space axes. The measuring signals of the sensors are fed to a control loop, which calculates control, or actuator signals from the measuring signals of the sensors, for devices generating magnetic fields.
The magnetic field to be compensated may be the terrestrial magnetic field, or may be generated by other current-carrying devices being in the surrounding.
Magnetic field compensation systems are for example used in connection with imaging systems using magnetic fields, for example in the case of scanning electron microscopes (SEM).
In case of the mentioned devices for generating magnetic fields, it may be a matter of a current-carrying conductor, in the easiest case. Generally, one assumes interfering fields having far field characteristics, i.e. such fields, whose field amplitude does not essentially change within the range of 5 m. This assumption for example is true for interferences by rail vehicles. If the interfering fields are homogeneous in the range of interest, the compensation fields should be homogeneous, also.
Pairs of so-called Helmholtz coils are preferably used for generating homogeneous compensation fields. At this, it is about two coils each being connected in the same direction, and having a distance to each other being equal to the half length of the edge (=coil diameter) (so-called Helmholtz condition).
Furthermore, pairs of Helmholtz coils are used, whose distance to each other is equal to one length of the edge. If one pair of Helmholtz coils is used for each of the three space axes, the pairs of coils form a cube-shaped cage around the location, at which one, or more interfering fields shall be compensated. In case of such a coil arrangement, there indeed are field inhomogeneities in the interior of the cage, but these are acceptable in the most cases of application.
A device for compensating magnetic fields is disclosed in U.S. Publication No 2005/019555A1 and has three coil pairs in a cage. The magnetic field to be compensated is measured and compensated, where an analog controller is used.
Systems are also available, with which only one coil per space axis is used for generating the compensation field, however the compensation region, i.e. the region in which a good compensation is achieved, is considerably smaller than in the case of Helmholtz coils.
Generally, one single magnetic field sensor is used for measuring the magnetic field at the place of interest. As an exception, there is a second sensor which is, however, used for diagnosis purposes. A single magnetic field sensor does not allow to detect, whether the magnetic field to be compensated is homogeneous, or inhomogeneous at the location of the object to be protected.
It is a further problem when compensating electromagnetic interfering fields that it cannot be measured directly at the location at which the interfering field is to be compensated, since the object to be protected against interfering fields generally is at this location.
A further problem arises, if two magnetic field compensation systems are arranged directly adjacent to one another. Then, undesired feedback effects may occur between the two systems.
There are problems with the control systems in that these control systems can generally be optimized to single application. An adjustment to control tasks that are quite different, such as upon changes in the control configuration, is as a rule not possible or only in a restricted manner possible and/or is to be implemented with great difficulties. Furthermore non-linear control systems which may have a better interference field compensation than linear control systems, generally can only be implemented with high costs. When control circumstances change, the whole control circuit or the control loop would have to be newly calculated, designed and/or changed. In most cases, the direct user is not a position to do so.